The proposed project examines the determinants of synaptic strength in the vertebrate nervous system using the neuromuscular junction as a model. The thin muscle used (snake transversus abdominis) is highly stereotyped and contains a variety of classes of motor synapses, all in one preparation. A new experimental technique (synaptic reconstitution) is utilized; the technique permits individual nerve terminal boutons from synapses of different classes to be studied in isolation. In addition, the boutons may be coupled to vacant muscle fiber endplate sites to form functional synapses. Two types of experiments are proposed. In the first, reconstructed and normal synapses will be studied and compared to assess the relationship between synaptic structure and function at the level of individual boutons. Experiments of the second type utilize synaptic reconstruction to form novel synapses comprising a bouton from one class of synapse coupled to the endplate site of another class. Together, these experiments will measure independently the pre- and postsynaptic influences on synaptic properties, and thereby assess the relative contributions of neuron and target cell towards determination of synaptic strength. Characterization of these relationships will increase knowledge of how strength of synapses is regulated so that the amount of transmitter released by the presynaptic neuron causes an appropriate conductance change in the target cell. This knowledge is essential in understanding why such regulation fails in diseases which attach pre- or postsynaptic cells throughout the nervous system.